Pedicle screw instrumentation is common in the lumbar spine and is gaining acceptance in the thoracic spine. The use of pedicle screw instrumentation in the spine has evolved over the last two decades. The initial use of pedicle screws began in the lumbar spine. As surgeons have become more comfortable with the complex anatomy required for accurate screw placement, the use of pedicle instrumentation has evolved to include their use in the thoracolumbar and thoracic spine. The impetus behind their increased use is a result of the many advantages that pedicle screw anchorage offers over traditional hook and rod constructs. Improved deformity correction and overall construct rigidity are two important advantages of pedicle screw instrumentation due its three-column control over the spinal elements. First, pedicle screw instrumentation obviates the need to place instrumentation within the spinal canal with its inherent risk of neurologic injury. Second, the placement of pedicle screws is independent of facet or laminar integrity and thus has been extremely useful in traumatic, neoplastic, and degenerative conditions. The benefit of pedicle screws in the thoracic spine has been tempered by the potential for catastrophic neurological or soft tissue injuries due to the close proximity of these structures. The narrow and inconsistent shape of the thoracic pedicles, especially in spinal deformity, makes their placement technically challenging. As a result, surgeons have employed a number of techniques to ensure the safe and efficacious placement of thoracic pedicle screws. Detailed anatomic landmarks used to determine pedicle location, intraoperative imaging including navigation, and neurophysiological monitoring are some of the techniques currently used by surgeons. The implementation of these techniques and a thorough understanding of the complex three-dimensional anatomy have allowed surgeons to successfully place thoracic and thoracolumbar pedicle screws.
Generally speaking, procedures for navigating the pedicle comprise the following steps: (1) decorticating the entry site using a burr and a high speed drill or a rongeur; (2) using an awl or a burr to penetrate the dorsal cortex of the pedicle and create a starter or pilot hole into the pedicle; (3) using a curved or straight pedicle probe to develop a path for the screw through the cancellous bone of the pedicle into the vertebral body (the process hereinafter referred to as “cannulation”). The advancement of the probe must be smooth and consistent and a sudden plunge suggests breaking out of the pedicle laterally. Furthermore, an increase in resistance indicates abutment against the pedicle or the vertebral body; (4) after cannulation, placing the pedicle sounding probe into the pedicle and then palpating the pedicle from within to make sure there is not a medial, lateral, rostral or caudal disruption in the cortex of the pedicle. Sound should also be used to determine that there is bone at the bottom of the pilot hole verifying that penetration of the ventral cortex of the vertebral body has not occurred; (5) after the pedicles have been probed, placing Steinman pins or K-wires bilaterally or unilaterally into the pedicles to confirm the trajectory and entry site, tapping the pedicle screw path if non-self tapping screws are used, and placing the permanent screws with the longest diameter that will not fracture the pedicle. The length of the screw can be determined by measuring the length of the Steinman pin/Kwire/pedicle probe from the pedicle entry site to a depth of 50-80% of the vertebral body; and (6) after pedicle screw placement, decorticating the transverse process and the lateral aspects of the facet joints, connecting the screw to a longitudinal construct, usually a rod or a plate, securing the screws, and placing bone graft on the previously fusion bed. During the entire process the advancement of the probe, the placement of the K-wires, and the ultimate advancement of the pedicle screws is monitored continuously via X-ray exposure or fluoroscopy. The present invention, as mentioned above, relates to cannulation during the pedicle screw procedure. One will now discuss in more detail the present invention, in particular, a pedicle punch that facilitates cannulation during pedicle screw implants.
In general a cannula is a flexible tube, which when inserted into the body may be used to withdraw fluid, insert medication or as in the present invention allow Lenke gearshifts, endoscopic probes, and the like to be inserted into the pedicle. Cannulae normally come with a trocar (a sharp pointed needle) attached which allows puncture of the body to get into the intended space.
Before the cannulation process begins, a posterior cortical breach first must be established on the pedicle, usually via any of the different methods as discussed in applicant's previous filed patent application, such as the burring method, the burning method or the punch method. A number of surgical instruments have been implemented to successfully cannulate thoracic pedicles. The two most common instruments include a gearshift device probe and a cervical curette. Another device that has been implemented with pedicle cannulation is Safe Path, a blunt-tipped, nonaggressive drill that seeks the cancellous portion of the pedicle. The pedicle punch of the present invention may be used with any of the aforementioned devices.
One will now discuss the utility of the present invention, for illustrative purposes one will discuss the present invention when used with a Lenke gearshift. A curved Lenke gearshift or a straight curette (3-0 cervical) may be used to probe or mature the intended screw path within the pedicle. The gearshift may be inserted via the cannula of the present invention. Once the neurocentral junction of the pedicle is reached (typically at a depth of 20 mm) the gearshift is removed and reinserted with the curved tip facing away from the surgeon. Positioning the pedicle probe tip medially assists in guiding the probe medially within the vertebral body. Next, the pedicle tract is palpated with a ball-tipped probe to verify the presence of a bony floor and an intact four-wall boundary. If a violation of bony integrity is noted at this point, redirecting the gearshift may be necessary in order to assure safe screw placement. The pedicle path is then tapped (preferably undertapped by 0.5-1.0 mm compared to the diameter of the selected screw) and the tract is repalpated with the ball-tipped probe to detect for any bony breaches. The pedicle screw is now inserted. Following screw insertion, intraoperative imaging is then performed to verify acceptable screw positioning. Triggered EMG testing may be used to evaluate for proper lower thoracic screw placement (T8-T12), while motor-evoked potential (MEP) monitoring assists in monitoring spinal cord function for all thoracic levels instrumented. Once all screws are placed and the applicable screws have been tested via triggered EMG the rod can be docked to complete the construct.
Even though most pedicle implant procedures are successful there is need for improvement. That is, after a posterior cortical breach is established, via the burring method or the burning method, the surgeon would first have to arrest the bleeding associated with these pedicle screw implant methods, before inserting one of the aforementioned probes.
Another drawback of the pedicle screw implant procedure is that the patient, surgeon, and medical staff are exposed to deleterious amounts of radiation, more specifically those deleterious amounts of radiation associated with fluoroscopy during the pedicle screw implant procedure. One way surgeons can protect themselves is with eyewear, thyroid shields, and lead aprons. However, studies with cadavers have shown that the surgeon's hands are still at a high risk of radiation exposure. In one study average fluoroscopy exposure time was 9.3 s per screw. and the average hand dose rate was 58.2 mrem/min. The internationally recommended maximum limit for annual hand radiation exposure is 50,000 mrem. In the same study a significant increase in hand dose rate was noted when placement of the screw was on the same side of the beam source as well as when a heavier cadaver was imaged.
Thoracic and thoracolumbar pedicle screw instrumentation is proving to be a safe and reliable method of obtaining rigid segmental fixation of the thoracic spine. A thorough understanding of the complex 3D spinal anatomy is required to safely place this type of instrumentation. The biomechanical benefits that are derived from using pedicle screw instrumentation in all forms of spinal pathology are the driving force behind more and more surgeons incorporating thoracic and thoracolumbar pedicle screw placement into their practices. Surgeons however, must be well versed in the placement of complex spinal instrumentation in order to accurately and safely use this method of instrumentation in all types of spinal disorders. The present invention, because of its unique design aids surgeons in the pedicle cannulation step via a pedicle punch that incorporates a cannula.